The present invention relates to electronic devices and, more particularly, to electronic devices including antennas for RF communication.
Antennas are well known devices that are used to convert electric current to electromagnetic waves and vice versa. The electromagnetic waves can be used to communicate information wirelessly from one electronic device to another.
An antenna includes an arrangement of electric conductors that radiates electromagnetic energy in response to an applied electric signal. Similarly, an antenna generates an electric signal at its terminals in response to an applied electromagnetic field.
Antennas are typically tuned to receive and transmit electromagnetic waves having a frequency that is near a resonant frequency (or center wavelength) of the antenna. The resonant frequency of an antenna is related to the electrical length of the antenna, which depends on the physical length of the antenna and on the dielectric material used in the antenna's construction. The dielectric material affects the velocity of signal propagation in the antenna. An antenna may be designed to transmit a narrow or broad range of frequencies. Furthermore, an antenna may be designed to transmit/receive electromagnetic waves in a directional fashion or an omni-directional fashion.
Many aspects of an antenna affect its performance and/or suitability for a particular application. For example, antennas are characterized by such attributes as gain, radiation pattern (directionality), impedance, efficiency, bandwidth and polarization.
Typically, an antenna has an electrical length that is about ¼ of a wavelength at the antenna's resonant frequency. Wireless telephones operate at a frequency of about 800 MHz to about 2.4 GHz. Some short range wireless RF communication devices operate in similar frequency ranges. For example, devices operating according to the Bluetooth standard use microwave radio frequency spectrum in the 2.4 GHz to 2.4835 GHz range. Accordingly, an antenna tuned to transmit/receive Bluetooth signals typically has an electrical length (λ/4) of about 3.1 cm.
Wireless telephones have commonly used simple vertical rods ¼ wavelength in length as antennas. While simple in design and providing omnidirectional transmission/reception (except for a null in the direction the rod points), these antennas have been used less widely as communication devices have become more and more miniaturized.
A compact, directional antenna design commonly used in wireless communication devices is a microstrip patch antenna. A microstrip patch antenna is a narrow bandwidth antenna that includes a conductive strip on one side of a dielectric layer and a conductive ground plane on the other side of the dielectric layer. Electric signals are transmitted to/from the conductive strip from feed points on the dielectric layer. The conductive strip can be formed by depositing metal on the dielectric layer and selectively etching the metal to form a desired pattern. The microstrip patch antenna preferentially radiates electromagnetic energy in a direction away from the ground plane.
While microstrip patch antennas can be miniaturized to fit on a handheld communication device, it is difficult to miniaturize such antennas further.